1. Field of the Invention
The present invention relates to a displacement measuring apparatus for calculating a moving amount of a probe brought into contact with an object to be measured in accordance with a change in oscillation frequency of an oscillator and, more particularly, to a displacement measuring apparatus wherein the service life of a battery for supplying a drive voltage to electronic components of the apparatus can be prolonged.
2. Description of the Related Art
A dial gauge shown in FIG. 1 is known as a portable displacement measuring apparatus for accurately measuring a very short distance of an object to be measured in units of 0.1 mm or 0.01 mm. Displacement measuring apparatus 1 represented by a dial gauge, is available to digitally display a moving amount from a reference position of a probe 2 on a display unit 3. A means for converting a moving amount of probe 2 into an electrical signal is exemplified by an oscillator built-in arrangement for detecting a change in oscillation frequency. More specifically, a core member is mounted on a shaft for supporting probe 2, and a pair of coils are wound around the core member. These coils are used to constitute a Colpitts oscillator. A moving distance of the core member, i.e. a moving distance of probe 2, is digitally calculated in accordance with a change in oscillation frequency f of the oscillator when the core member interlocked with probe 2 is moved in the pair of coils. A calculation result is digitally displayed on display unit 3.
FIG. 2 is a block diagram showing electronic components built into displacement measuring apparatus 1. Frequency f of a Colpitts oscillator 4 having coils which receive the core member connected to probe 2 is counted by counter 5. Oscillation frequency f counted by counter 5 is input to arithmetic control unit 6 comprising a microcomputer or the like. Arithmetic control unit 6 calculates a moving amount of probe 2 in accordance with a change in frequency f input thereto. The calculated moving amount is digitally displayed on display unit 3.
Battery 7 is stored in a casing of displacement measuring apparatus 1 to apply drive voltage V.sub.D to electronic components 4, 5, 6, and 3. For example, if an initial voltage of battery 7 is set to be about 3.3 V (rated voltage V.sub.B =3.0 V), output voltage V.sub.O of battery 7 is boosted by voltage booster 8 to a voltage of, e.g. 4 to 6 V. The boosted voltage is decreased to drive voltage V.sub.D of, e.g. about 3.6 V by voltage stabilizing circuit 9. Drive voltage V.sub.D is always controlled to be a predetermined voltage by voltage stabilizing circuit 9 and is supplied to oscillator 4, counter 5, arithmetic control unit 6, and display unit 3.
Voltage booster 8 and voltage stabilizing circuit 9 are used in the above arrangement due to the following reason. Output voltage V.sub.O of battery 7 exhibits initial voltage of 3.3 V when battery 7 is replaced with a new one. However, when displacement measuring apparatus 1 is used for a long period of time, power consumption of the respective electronic components causes a decrease in output voltage V.sub.O. In order to maintain high measurement precision in the displacement measuring apparatus, high stability of oscillation frequency f of oscillator 4 is required. Therefore, high stability of drive voltage V.sub.D applied to oscillator 4 is also required.
If rated voltage V.sub.B of battery 7 has a value sufficiently larger than that of drive voltage V.sub.D of the electronic components, voltage booster 8 need not be used.
When battery 7 is mounted in a battery holder without arranging a power switch so as to reduce manufacturing cost, drive voltage V.sub.D is immediately applied to electronic components 4, 5, 6, and 3.
The following problem occurs in the displacement measuring apparatus for applying drive voltage V.sub.D to the respective electronic components after output voltage V.sub.O of battery 7 is stabilized to the predetermined voltage such as 3.6 V by voltage booster 8 and voltage stabilizing circuit 9. Voltage stabilizing circuit 9 must be used to decrease the DC voltage so as to obtain stable drive voltage V.sub.D. Power consumption of voltage stable circuit 9 is equivalent to a total current value of current values of drive voltage V.sub.D applied to the electronic components. If the number of electronic components which receive drive voltage V.sub.D from voltage stable circuit 9 is increased, power consumption of voltage stable circuit 9 is also increased. As a result, the service life of battery 7 is undesirably shortened.
In particular, in the displacement measuring apparatus (FIG. 2) wherein output voltage V.sub.O from battery 7 is boosted by voltage booster 8 and then decreased to the predetermined voltage by voltage stabilizing circuit 9, power consumption of voltage booster 8 is large, and therefore, the service life of the battery is further shortened.